The largest US refinery strike in 35 years entered its fourth week as workers at 12 refineries accounting for one-fifth of national production capacity were walking picket lines as of 12 a.m. on Sunday, according to union officials.

A total of 6,550 members of the United Steelworkers union at 15 plants, including the 12 refineries, are involved in the work stoppage that began on February 1 when talks for a new three-year contract between the USW and lead oil company negotiator Shell Oil Co stopped.

Talks were resumed but have halted again after nearly reaching an agreement on Friday, said sources familiar with the negotiations.

After the latest breakdown between the two sides, Steelworkers leaders targeted Shell, which is the US arm of Royal Dutch Shell Plc, calling workers out at a chemical plant and three refineries in the company's Motiva Enterprises joint-venture with Saudi Aramco .